In 2012, materials scientist John Rogers at the University of Illinois at Urbana-Champaign unveiled a range of biodegradable silicon chips that could monitor temperature or mechanical strain, radio the results to external devices, and even heat up tissue to prevent infection. Some of those chips relied on induction coils to draw wireless power from an external source. But wireless power transfer is problematic for devices that need to go deep within tissue or under bone.

And the components that receive the power are pretty complicated, which adds to the bulk. So to provide a tidier solution, Rogers and colleagues created a fully biodegradable battery that uses metal foils, saline solution, and biodegradable polymers. “Almost all of the key building blocks are now available” to produce self-powered, biodegradable implants, Rogers tells Nature.

The device, Nature explains, consists of anodes of magnesium foil and cathodes of iron, molybdenum or tungsten -- metals with ions that are biocompatible at low concentrations. The electrolyte between the two electrodes is a phosphate-buffered saline solution, and the whole system is packaged in polyanhydride, a biodegradable polymer.

Currents and voltages vary depending on the metal, but at least one version produces a steady 2.4 milliamps of current. A tiny battery with a surface area of 0.25 square centimeters and a thickness of 1 micrometer could realistically power a wireless sensor for up to a day (but not much longer).

The implant dissolves completely (pictured above) in three weeks -- but not before dispensing drugs or monitoring vital signs, and then wirelessly relaying data. Once dissolved, the battery releases less than 9 milligrams of magnesium, a concentration that’s unlikely to cause problems in the body, according to Rogers.

Janet Fang has written for Nature, Discover and the Point Reyes Light. She is currently a lab technician at Lamont-Doherty Earth Observatory. She holds degrees from the University of California, Berkeley and Columbia University. She is based in New York.
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